1. Field of the Invention
The invention relates to the rotor blades of the fan of a turbojet engine.
2. Description of the Prior Art
Modern turbojet engines of the bypass type usually have a compressor assembly, which is termed a fan, comprising at least one stage of rotor blades at the outlet of which the compressed air is divided into two flows: a primary flow which enters the subsequent compression stages before passing into a combustion chamber to generate a hot gas flow, and a secondary flow which enters an annular duct, termed the by-pass duct, and which, in the absence of any heating, particularly in civil turbojet engines, constitutes a cold flow . The fan thus incorporated is termed a ducted fan. The aerodynamic efficiencies of the fan are directly related to the sealing achieved between the tips of the rotor blades and the corresponding fixed inner wall of the fan casing. In order to avoid any damage having serious consequences as a result of accidental contact between the tip of a rotor blade and the associated fixed wall, which may occur due to various causes which may also be accidental (ingestions, for example) or originate from other structural or functional factors (aging, expansion, deformation, for example), the inner wall of the casing facing the blade tips usually has a wear and seal lining, termed abradable.
The invention is concerned with improving the results which have been observed during such contacts between a fan blade tip and the abradable lining of the associated casing. Indeed, one solution previously adopted with a view to ensuring a seal between blade tip and casing, while endeavouring to obtain acceptable operation during frictional contacts, consists of machining at the end of the aerofoil portion of the blade a thin tongue over the entire width of the blade profile, the tongue being intended to ensure good penetration into the abradable lining. FIGS. 1a, 1b and 1c of the attached drawings show an example of this known construction. The tongue 1 of the aerofoil portion 2 of a blade 3 faces the abradable lining 4 of a casing 5. However, it has been observed in this construction that as a result of the contacts between the tongue 1 and the abradable lining 4 the wear of the lining 4 exhibits irregularities, grooves and scorch marks, which seem due to the fact that chattering and bottoming phenomena occur during these contacts.
French patent specification No. A 2 459 363 also addresses certain problems met with during the rubbing of the blade tips against the wall of the casing, and more precisely seeks to achieve axial stabilization of the blades through a preferential orientation of the resultant force developed during contact. At the blade tip, a serrated profile associated with a particular geometry is obtained by means of recesses made on the concavely curved face of the blade.
However, this solution does not solve satisfactorily the problem mentioned earlier and requires, in addition, the making of a complex profile, which the invention seeks to avoid in providing a simple and better solution than is hitherto known.